Electronic devices utilizing wireless digital communications techniques have become increasingly prevalent in modern society. Pagers, cellular and cordless telephones, wireless computer networks, and digital television receivers are but a few examples of technologies that are becoming more and more commonplace in everyday life. Meanwhile, the increasing popularity of portable and miniaturized electronic devices is forcing designers to meet strict performance demands while simultaneously reducing circuit board size and power consumption. Not only are designers faced with the objective of reducing circuit size and power consumption but they must also address the demand for efficient allocation and use of the frequency spectrum in view of the increase in wireless traffic over a fixed number of allocated frequency bands.
One characteristic of modulated communications signals which places constraints on the efficient use of frequency spectrum is the presence of what is known as sideband energy. Sideband energy is a byproduct of signal modulation and amplification processes. Sideband energy "lies" generally adjacent to the frequency band of the desired signal undergoing modulation and results in the creation of unwanted interference on adjacent frequencies.
One technique known in the art of digital signal communications to reduce the amount of excess sideband energy generated during modulation of a digital signal is to filter the digital signal before modulating it. By smoothing the transitions between digital high and low levels, less interfering RF energy is generated at frequencies adjacent to the desired modulated frequency signal band. Consequently, this technique allows communications channels to be placed closer together with less cross-channel interference, thereby allowing more efficient utilization of a limited frequency spectrum and increased performance of radio traffic on adjacent channels. Additionally, smoothing the digital signal transitions before modulation reduces the need for post-modulation analog filtering at RF frequencies, which is otherwise comparatively delicate and expensive.
While smoothing a digital waveform before modulation is an effective technique for increasing radio performance, prior art methods of implementing such techniques have suffered several limitations. Analog filtering methods require additional space on a circuit board, are typically less precise and flexible, and may even require amplifier components, further adding to circuit size and cost. One prior art digital technique of shaping a binary signal waveform is to apply the signal to a digital filter. Conventional digital filtering has the advantages of added precision and flexibility, compared to analog methods, and may be integrated into multifunctional ICs to eliminate the need for additional board space. However, the multipliers, adders, and memory components commonly required by a digital filter implementation necessitate use of a significant number of logic gates to implement, thereby adding complexity to IC design and increased circuit power consumption.
Accordingly, it is an object of this invention to provide an apparatus for the shaping of binary waveforms which can be implemented simply, inexpensively, and with minimal increase in power consumption. Rather than fully implementing a digital filter to process the binary input signal, the present invention stores a predetermined sequence of values, which are output consecutively, either forwards or backwards, in accordance with the state of the binary input signal. By storing only samples corresponding to a transition between binary states, the complexity and memory requirements of the resulting circuit are greatly reduced.
It is another object of this invention for the predetermined sequence of values to correspond to samples of a filtered binary state transition, such that the output of the apparatus is equivalent to the binary input signal after processing by a specifically designed filter, such as a raised cosine or Gaussian filter, while requiring less complexity than other methods of implementing filter designs.
Yet another object of this invention is the inclusion of an enable input, such that the apparatus may be alternatively enabled or disabled, such that when the enable input is placed in the disabled state, the apparatus outputs a fixed predetermined value.
These and other objects of the present invention will become apparent in view of the present specification and drawings.